The present invention relates to a device for medical imaging and diagnosis, and in particular, to the use of elastography for the evaluation of cardiac health.
Elastography is a new imaging modality that reveals the stiffness properties of tissues, for example, axial strain, lateral strain, Poisson""s ratio, Young""s modulus, or other common strain and strain related measurements. The strain measurements may be collected over an area and compiled as a two-dimensional array of data, which may then be mapped to a gray scale to form a strain xe2x80x9cimagexe2x80x9d.
In xe2x80x9cquasi staticxe2x80x9d elastography, two conventional images of the tissue are obtained using ultrasound, computed tomography (CT), or magnetic resonance imaging (MRI). The first image provides a base line of the tissue at a given state of compression or distention and the second image is obtained with the tissue under a different compression or distention. The tissue may be compressed by an external agency such as a probe or the like or may be compressed by its own muscular action, for example, in the case of the heart, or by movement of adjacent organs. Displacement of the tissue between the two images is used to deduce the stiffness of the tissue. Quasi-static elastography is thus analogous to a physician""s palpation of tissue in which the physician determines stiffness by pressing the tissue and detecting the amount that the tissue yields under this pressure.
In xe2x80x9cdynamicxe2x80x9d elastography, a low frequency vibration is applied to the tissue and the tissue vibrations accompanying the resulting elastic wave are measured, for example, using ultrasonic Doppler detection.
Elastography has recently been investigated as a method of detecting cardiac dysfunction. Normal, periodic myocardial thickening, associated with proper heart function, may be revealed in the strains shown in an elastographic image. Tissue ischemia or infarction may thus be detected as a reduction of myocardial thickening.
Despite the promise of elastography for cardiac evaluation, effective methods for displaying myocardial strain and of relating elastographic measurements to cardiac disease have not yet been developed.
The present invention provides an improved method and apparatus for producing elastographic images of the heart to detect cardiac disease.
The invention includes in its several embodiments: a visually improved mapping of the two dimensions of strain (direction and sign) to a color scale, an area cursor quantifying strain measurements within predefined regions, and a quantitative metric of cardiac function comparing different predefined heart regions to reduce operator variability in the assessment of cardiac disease.
Specifically, the present invention provides an elastography apparatus including a medical imaging system, operating on in vivo tissue, to provide at least a two-dimensional array of strain values related to points in the tissue. Each strain value has a magnitude and sign indicating an amount of strain at a point and whether the strain is compression or distension, respectively. The apparatus further includes an image generator mapping the array of strain values to colors at pixels in an image such that brightness of the colors varies monotonically with absolute value (magnitude) strain value and hue of the colors is related to strain value sign.
Thus, it is one object of the invention to provide a visually intuitive color mapping for strain by independently mapping two dimensions of strain to brightness and hue.
Zero absolute value strain may map to black.
It is another object of the invention to visually de-emphasize regions of low strain.
The compressive tissue strain may map to warm hues and distensive tissue strain may map to cool hues.
It is thus another object of the invention to provide a clear visual distinction between compressive and distensive strains.
In one embodiment, the signal processing circuitry may provide a second image of the heart tissue indicating relatively time invariant tissue quantities.
Another object of the invention can be to provide a separate image to serve as a point of reference for the strain image.
The two images may be side-by-side on a single display device and a first and second movable cursor may be superimposed on corresponding regions of the images.
The two images may also be superimposed on a single display device with a cursor used to navigate about the strain image, with the wall location identified by the gray-scale ultrasound image.
Thus, it is another object of the invention to simplify navigating about the strain image. One of the cursors can be located on a region identified in the conventional image to locate the corresponding region in the strain image.
The cursor may define a region of interest and the signal processing circuitry may provide a quantitative display of strain of tissue within the region of interest.
Thus, it is another object of the invention to provide a quantitative and less observer dependent measurement of tissue strain.
The apparatus may include a means for identifying a phase of the beating heart and the quantitative display may be related to the phase of the beating heart. For example, the quantitative display may provide an indication of strain of the tissue within the region of interest at the end of the systolic phase or the end of the diastolic phase of the beating heart.
It can thus be another object of the invention to provide a robust repeatable measurement of strain that may be useful for generating a standardized index for cardiac function.
The apparatus may provide strain measurements at several predefined regions in the heart tissue. The quantitative display may then be a comparison of strains in these regions.
Thus, it is another object of the invention to provide a standardized index for cardiac function that makes use of a multi-point quantitative assessment, difficult for an unassisted observer.
These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention.